A molecular simulation study of the structure and tribology of polymer brushes: Comparison of behavior in theta and good solvents

Implicit solvent Brownian dynamics simulations of the structure and tribology of opposing polymer-brush covered surfaces have been carried out as a function of surface separation and solvent quality. Consistent with experiment, shear forces were found to be greater under theta solvent conditions than in a good solvent at equal relative separations (normalized by the respective height of the brushes in theta and good solvents). Much higher relative compression is required before the onset of significant shear forces in good solvent compared to theta solvent. The dependence of shear force for a given relative separation on solvent quality can be accounted for by differences in interpenetration of the brushes. When compared as a function of absolute surface separation, greater interpenetration and greater shear force are observed at large separations for the brushes in good solvent than in theta solvent, consistent with the greater brush height in good solvent. At shorter separations, corresponding to moderate to high compression, brush-brush interactions result in significant deformation of the brushes. In this regime, greater interpenetration and greater shear forces are observed in theta solvent at a given separation, in qualitative agreement with experiment.     

溶剂浮选法分离富集葛根中大豆甙元的研究

采用溶剂浮选法分离富集葛根中的大豆甙元。考察了浮选溶剂、氮气流速、试液pH、浮选时间及电解质（KC1）等因素对浮选效率的影响,优选出最佳浮选条件;对最佳条件下的浮选效果进行了评价,并与溶剂萃取法进行了对照,前者明显优于后者。     

Broadening of peaks eluted before the solvent in capillary GC Part II: The role of phase soaking

Summary Phase soaking is a solvent effect which tends to reconcentrate peaks eluted after and to broaden peaks eluted before the solvent. The principles of the phase soaking effect on peaks eluted before the solvent are discussed. The broadening effect is distinguished from the broadening effect occurring in the flooded column inlet by partial solvent trapping. It was found that in most cases broadening by partial solvent trapping strongly predominated over broadening by phase soaking. Phase soaking was noticeable only in the neighbourhood of the solvent peak.Phase soaking does not broaden peaks eluted before the solvent as much as it re-concentrates those eluted after it. The two phase soaking effects on the front and the rear of the solvent band (that is, of the soaked zone) differ strongly from each other.Peak broadening by phase soaking is negligible for non-trapped components, because such solutes start their chromatography before a significant quantity of solvent enters the column. Phase soaking only broadens solute bands which were retained by the solvent in the column inlet, that is, bands already broadened by partial solvent trapping.     

How does solvent molecular size affect the microscopic structure in polymer solutions?

Monte Carlo simulation has been used to investigate the effects of linear solvent molecular size on polymer chain conformation in solutions. Increasing the solvent molecular size leads to shrinkage of the polymer chains and increase of the critical overlap concentrations. The root-mean-square radius of gyration of polymer chains (R(g)) is less sensitive to the variation of polymer concentration in solutions of larger solvent molecules. In addition, the dependency of R(g) on polymer concentration under normal solvent conditions and solvent molecular size is in good agreement with scaling laws. When the solvent molecular size approaches the ideal end-to-end distance of the polymer chain, an extra aggregation of polymer chains occurs, and the solvent becomes the so-called medium-sized solvent. When the size of solvent molecules is smaller than the medium size, the polymer chains are swollen or partially swollen. However, when the size of solvent molecules is larger than the medium size, the polymer coils shrink and segregate, enwrapped by the large solvent molecules.     

Scaling and structural properties of a polymer in a simple solvent: A study based on integral equations

We present a statistical mechanical theory for polymer–solvent systems based on integral equations derived from the polymer Kirkwood hierarchy. Integral equations for pair monomer–monomer, monomer–solvent, and solvent–solvent correlation functions yield polymer–solvent distribution, chain conformation in three dimensions, and scaling properties associated with polymer swell and collapse in athermal, good, and poor solvents. Variation of polymer properties with solvent density and solvent quality is evaluated for chains having up to 100 bonds. In good solvents, the scaling exponent v has a constant value of about 0.61 at different solvent densities computed. For the athermal solvent case, the gyration radius and scaling exponent decrease with solvent density. In a poor solvent, the chain size scales as N^v with the value of the exponent being about 0.3, compared with the mean field value of ⅓. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3025–3033, 1998     

溶剂浮选法分离富集茶叶中茶多酚的研究

采用溶剂浮选法分离富集了茶叶中的茶多酚。考察了浮选溶剂、氮气流速、试液pH及浮选时间等因素对浮选效率的影响,优选出最佳浮选条件;对最佳条件下的浮选效果进行了评价,并与溶剂萃取法进行了比对,前者明显优于后者。     

Solvent properties governing protein partitioning in polymer/polymer aqueous two-phase systems

Distribution coefficients of various proteins were measured in aqueous Dextran-Ficoll, Dextran-PES, and Ficoll-PES two-phase systems, containing 0.15M NaCl in 0.01 M phosphate buffer, pH 7.4. The acquired data were combined with data for the same proteins in different systems reported previously and known solvatochromic solvent properties of the systems to characterize the protein-solvent interactions. The relative susceptibilities of proteins to solvent dipolarity/polarizability, solvent hydrogen bond acidity, solvent hydrogen bond basicity, and solvent ability to participate in ion-ion and ion-dipole interactions were characterized. These parameters, which are representative of solute-solvent interactions, adequately described the partitioning of the proteins in each system. It was found that the relative susceptibilities of proteins to solvent dipolarity/polarizability are interrelated with their relative susceptibilities to solvent hydrogen bond acidity and solvent hydrogen bond basicity similarly to those established previously for small nonionic organic compounds.     

Chemical reaction in binary mixtures near the critical region: thermal decomposition of 2,2'-azobis(isobutyronitrile) in CO2/ethanol

The effects of pressure and of the composition of the CO2/ethanol mixed solvent in the critical region on the kinetics of the decomposition of 2,2'-azobis(isobutyronitrile) (AIBN) were studied at 333.15 K. The rate constants (kd) in the mixed solvent far from the critical point and in liquid n-hexane and ethanol were also determined for comparison. It was found that kd is very sensitive to pressure in the mixed solvent near the critical point. However, in the mixed solvent outside the critical region kd is nearly independent of pressure. Interestingly, kd in the mixed solvent in the critical region can be higher than that in ethanol at the same temperature, suggesting that no significant enhancement in the reaction rate by a small pressure change in the critical region of the mixed solvent can be achieved by changing the composition of the liquid solvent in the traditional way. Transition-state theory can predict kd in the mixed solvent far from the critical point and in the liquid solvents well. However, it cannot predict kd in the mixed solvent in the critical region. The special intermolecular interaction between the solvent and the reaction species may contribute to this interesting phenomenon. This work also shows that if pure CO2 or ethanol are used as solvents, the reaction cannot be carried out in the critical region of the solvents at the desired temperature, while it can be conducted in the critical region of the mixed solvent of suitable composition, where the solvent is highly compressible.     

Effective potentials between nanoparticles in suspension

Results of molecular dynamics simulations are presented for the pair distribution function between nanoparticles in an explicit solvent as a function of nanoparticle diameter and interaction strength between the nanoparticle and solvent. The effect of including the solvent explicitly is demonstrated by comparing the pair distribution function of nanoparticles to that in an implicit solvent. The nanoparticles are modeled as a uniform distribution of Lennard-Jones particles, while the solvent is represented by standard Lennard-Jones particles. The diameter of the nanoparticle is varied from 10 to 25 times that of the solvent for a range of nanoparticle volume fractions. As the strength of the interactions between nanoparticles and the solvent increases, the solvent layer surrounding the nanoparticle is formed which increases the effective radii of the nanoparticles. The pair distribution functions are inverted using the Ornstein-Zernike integral equation to determine an effective pair potential between the nanoparticles mediated by the introduction of an explicit solvent.     

Conformation of a polymer chain in solution: an exact density expansion approach

The conformation of a polymer chain in solution is intrinsically coupled to the thermodynamic and structural properties of the solvent. Here we study such solvent effects in a system consisting of a flexible interaction-site n-mer chain immersed in a monomeric solvent. Chain conformation is described with a set of intramolecular site-site probability functions. We derive an exact density expansion for these intramolecular probability functions and give a diagrammatic representation of the terms contributing at each order of the expansion. The expansion is tested for a short hard-sphere chain (n=3 or 4) with site diameter sigma in a hard-sphere solvent with solvent diameter D. In comparison with Monte Carlo simulation results for 0.2< or =D/sigma< or =100, the expansion (taken to second order) is found to be quantitatively accurate for low to moderate solvent volume fractions for all size ratios. Average chain dimensions are predicted accurately up to liquidlike solvent densities. The hard-sphere chains are compressed with both increasing solvent density and decreasing solvent size. For small solvent (D相似文献   

Solvent effects on two-dimensional molecular self-assemblies investigated by using scanning tunneling microscopy

Self-assembly structures investigated by using scanning tunneling microscopy (STM) at liquid/solid interface have been a topic of broad interest in surface science, molecular materials, molecular electronics. The delicate balance among the adsorbate–solvent, adsorbate–adsorbate, solvent–solvent interactions would give rise to the coadsorption or competitive deposition of solvent with adsorbate. The solvents at the interface enable dynamic absorption and desorption of the adsorbates leading to the controlled assembly of the molecular architectures. The solvent-induced polymorphism, coadsorption effect, as well as solvent effects on chirality and electronic structures are discussed in this report in view of the polarity, solubility and viscosity of the solvent, the hydrogen bonding formation between solute and solvent, and the solvophobic and solvophilic effects. The systematic studies on the solvent effects would shed light on better control of assembly structures for design of new molecular materials and molecular electronics.     

用气相色谱法研究乙酸乙烯酯不对称氢甲酰化反应中溶剂的影响

用毛细管气相色谱、色－质谱和旋光色散及圆二色性谱仪等方法对乙酸乙烯酯在非极性和极性溶剂中的不对称氢甲酰化反应产物进行分离和鉴定。实验结果表明,在非极性溶剂中反应的收率、选择性、光学收率ｅ．ｅ．值（ｅｎａｎｔｉｏｍｅｒｉｃｅｘｃｅｓｓ）均比极性溶剂中的结果为好。由此探讨了不对称氢甲酰化反应中溶剂的影响。     

溶剂浮选分离富集麻黄草中有效成分

采用溶剂浮选法分离富集麻黄草中的有效成分。考察了浮选溶剂、氮气流速、试液pH、浮选时间及电解质NaCl等因素对浮选效率的影响,优选出最佳浮选条件;对最佳条件下的浮选结果进行了评价,并与溶剂萃取法进行了对照;对麻黄草有效成分的浮选过程进行了初步探讨,浮选过程符合一级动力学方程。     

Mechanism of decoloration by solvent of thermally degraded poly(vinyl chloride)

The mechanism of decoloration of thermally degraded poly(vinyl chloride (PVC)) by solvents has been investigated systematically. The main results obtained are as follows. Good solvents for PVC, especially tetrahydrofuran, methyl ethyl ketone, and dioxane are effective for decoloration. The solvent peroxide which is formed by autoxidation of solvent contributes to decoloration. The number of double bonds in degraded PVC decreases as the decoloration proceeds and at the same time the solvent peroxide existing in solvent is consumed. Moreover, the existence of solvent fragments in decolored PVC is recognized. From these results, it is most reasonable to conclude that the decoloration mechanism is as follows: the solvent partially is changed to a solvent peroxide by autoxidation, and the solvent peroxide reacts with polyene double bonds of degraded PVC and breaks down conjugated double bonds, and consequently degraded PVC is decolored.     

Solvent density effects on the solvation behavior and configurational structure of bare and passivated 38-atom gold nanoparticle in supercritical ethane

In exploring the effects of solvent density on the mode and the degree of solvation of the bare and passivated 38-atom gold particle in supercritical ethane, we have extended the molecular dynamics simulations of the system, reported previously,(34) to cover a range of isotherms in the T > T(c) regime, where T(c) is the critical temperature of the solvent. Consonant with our previous observations, the modes of solvation of the bare and the passivated particle, deduced from the radial distribution of the solvent about the metal core center of mass, are found to be vastly different from each other at all solvent densities: while the molecules solvating the bare particle form a well-defined, two-region layer around it, those solvating the passivated particle are loosely dispersed in the passivating layer. For the bare particle, the degree of solvation (vartheta) as a function of solvent density passes through a maximum occurring in the close vicinity of the critical point, consistent with our previous results and in agreement with Debenedetti's theoretical analysis,(22,23) which predicts a solvation enhancement effect in the critical region for systems where the unlike solvent/solute interaction is much stronger than the solvent/solvent interaction. Taking the degree of solvation (vartheta) as a measure of solvent quality, we have investigated how the solvent quality would vary along the solvent-density isotherms. In the solvent-density regime rho > rho(c), the solvent quality is found to be a decreasing function of the density as a result of progressive dominance of the excluded volume effect over the attractive particle/solvent interactions. The particle/solvent affinity is greatly reduced in the presence of the passivating layer, resulting in considerable shrinkage of the good-solvent-quality domain in the supercritical regime. The solvent environment and the presence of the passivating chains produce significant disorder in the equilibrium structure assumed by the nanoparticle core.     

Ab initio QM/MM study of excited state electron transfer between pyrene and 4,4′‐bis(dimethylamino)‐diphenylmethane with different solvent systems: Role of hydrogen bonding within solvent molecules

The exciplex is a charge transfer species formed in the process of electron transfer between an electron donor and an electron acceptor and hence is very sensitive to solvent polarity. In order to understand the role of solvent in exciplex formation between pyrene (PY) and 4,4′‐bis(dimethylamino)diphenylmethane (DMDPM), we used two types of solvent approximations: an implicit solvent model and an explicit solvent model. The difference in energies between the excited and the meta‐stable Frank–Condon state (ΔE) of the structures were assumed to correspond to the emission maximum of the exciplex in different solvents. The ΔE values show the trend of stabilization of the exciplex with an increase in solvent polarity. This trend in stabilization is substantially more prominent in the explicit solvent model than that with the implicit solvent model. The ΔE value obtained in methanol reflects equal stabilization compared to that in a more polar solvent, N,N‐dimethylformamide. This extra stabilization of the exciplex may be explained on the basis of the H‐bonding capability of the protic solvent, methanol. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Solvent effect on the absorption spectra of coumarin 120 in water: A combined quantum mechanical and molecular mechanical study

The solvent effect on the absorption spectra of coumarin 120 (C120) in water was studied utilizing the combined quantum mechanical∕molecular mechanical (QM∕MM) method. In molecular dynamics (MD) simulation, a new sampling scheme was introduced to provide enough samples for both solute and solvent molecules to obtain the average physical properties of the molecules in solution. We sampled the structure of the solute and solvent molecules separately. First, we executed a QM∕MM MD simulation, where we sampled the solute molecule in solution. Next, we chose random solute structures from this simulation and performed classical MD simulation for each chosen solute structure with its geometry fixed. This new scheme allowed us to sample the solute molecule quantum mechanically and sample many solvent structures classically. Excitation energy calculations using the selected samples were carried out by the generalized multiconfigurational perturbation theory. We succeeded in constructing the absorption spectra and realizing the red shift of the absorption spectra found in polar solvents. To understand the motion of C120 in water, we carried out principal component analysis and found that the motion of the methyl group made the largest contribution and the motion of the amino group the second largest. The solvent effect on the absorption spectrum was studied by decomposing it in two components: the effect from the distortion of the solute molecule and the field effect from the solvent molecules. The solvent effect from the solvent molecules shows large contribution to the solvent shift of the peak of the absorption spectrum, while the solvent effect from the solute molecule shows no contribution. The solvent effect from the solute molecule mainly contributes to the broadening of the absorption spectrum. In the solvent effect, the variation in C-C bond length has the largest contribution on the absorption spectrum from the solute molecule. For the solvent effect on the absorption spectrum from the solvent molecules, the solvent structure around the amino group of C120 plays the key role.